1. Field of the Invention
The invention relates to a clocked power supply unit with a low-voltage output for direct current comprising at least one secondary winding of a transformer, one rectifier which is connected (indirectly or directly) to the secondary winding, two outputs at which a predefined output voltage is able to be tapped, and a shunt which is configured to provide a voltage signal or a current signal which is proportional to the output current.
2. Description of the Related Art
A power supply unit generally comprises an input stage into which the supply voltage (input voltage) is fed as alternating voltage, i.e., from the alternating current network or a semiconductor bridge and is supplied to a primary winding of a transformer. The output stage of a power supply unit comprises the secondary winding of the transformer and a rectifier. Between the secondary winding and the rectifier and/or the shunt, naturally a further stage may be interposed, i.e., a step-down converter or “buck” regulator.
Such power supply units are often used in switchgear cabinets for electrical systems to provide a predefined supply voltage (control voltage) to a control electronics unit, such as 24 V DC. In principle, however, direct voltages of up to 120 V come within the low-voltage range, while below 60 V DC is referred to as harmless voltages that do not require any particular shielding.
If power supply units cited in the introduction are used in switchgear cabinets for electrical systems, the distribution of the output voltage as control voltage in the switchgear cabinet occurs via insulated individual conductors that are laid in cable channels together with signal lines. Frequently, up to 20 individual conductors are laid in a cable channel so that the temporary removal of a cable for measuring purposes, i.e., for measuring the output current of the power supply unit, is very complex.
When testing a switchgear cabinet during the production thereof in the workshop, when an error search is performed at the installation site of the switchgear cabinet during the start-up of the electrical system supplied by the switchgear cabinet, when performing maintenance operations or an error search during operation of the electrical system, the measurement of the supply current delivered by the power supply unit, generally 24 VDC, is a significant feature of the diagnosis and error limitation.
The measurement of the power consumption of a fully wired electrical system is complex and time-consuming because either the system has to be switched off or the conductor to be measured has to be cut off from the power supply and an ammeter has to be looped in. Due to time constraints during the error search, time-consuming tests are postponed and only performed when a first rapid diagnosis provides no indication of the error. If a simple and rapid error search were available to the service technician, defects that are present in the control electronics system and are able to be identified by a greater supply current consumption (such as in proximity switches, or light barriers) could be rapidly identified and eliminated during the error search.
Only the following methods for identifying errors by determining the power consumption have been available hitherto.
A service technician measures the control voltage via of a multi-meter or installs a voltage-dependent display in the output current circuit. The display is able to indicate via an LED, for example, the presence of the desired output voltage and/or control voltage of, for example, 24 V and/or via an LED when a voltage threshold, such as 21 V, is fallen below. These two conventional methods permit only a very indirect assessment of the power consumption of the control voltage because it is only possible to identify large increases in the power consumption that overload the supplied power supply such that the output voltage drops.
A conventional method for direct measurement of the output current comprises disconnecting the output terminal from the power supply unit and looping in an ammeter. This requires the electrical system to be switched off or to be partially switched off, because the looping in is only able to occur in the current-less state. Otherwise, when the contacts “bounce” it results in multiple brief stoppages before the ammeter is looped in. These undesired stoppages may cause an additional unforeseeable reaction of the electrical system, such as reset functions, multiple activation of actuators and the like, which is not generally accepted by the service technician (and by the system operator).
Instead, a clip-on ammeter that is suitable for direct current could also be looped into the control voltage output of the power supply unit. This is theoretically possible even when the system is in operation because a cable connection does not have to be disconnected. The drawback with this conventional method is that the conductors that are frequently tightly laid in a cable channel are inaccessible and/or the threading of the conductor through the tight layout is difficult. The conductor, therefore, during operation has to be pulled sufficiently far out of the cable channel for the clip-on ammeter to fit through with the magnetic clip. If the cable of the conductor is laid without movement loops, i.e., without an excess length, it is often impossible to pull it out. The system operators, however, generally do not, in any case, permit the conductor to be pulled during operation, resulting in the system having to be definitely switched off.
A further method for determining the output current is that of an electronically generated signal that is proportional to the output current and is provided at a test point. This conventional method generally requires amplification of a small voltage signal that is delivered by a current measuring resistor, also called a shunt, and provided to the customer at a measurement output of the power supply unit. Thus, for example, a voltage that by the ratio 1V=1 A represents a measurement of the output current may be provided at a measurement output. In order to achieve sufficient accuracy, expensive operation amplifiers, i.e., those with low offset voltage, are used and a similarly expensive reference voltage source, with a low tolerance for temperature and ageing. In any case, a relatively small additional error may occur with expensive operation amplifiers and reference voltages that further increase the inaccuracy of the shunt.
The installation of current measuring devices in the power supply unit might also be possible, where the current measuring devices measure the output current and either display it locally on the power supply unit or transmit it to a superordinate controller, i.e., via a data bus. The method of the installed current measuring device, however, is too complex and costly for many applications.